The present invention relates generally to reproducing of a record carrier using magnetic means, and more particularly to a magnetoresistive device. The present invention is suitable, for example, for a read head for use with a hard disc drive (referred to as “HDD” hereinafter).
Available electronic information content has explosively increased with the recent rapid technology development, as in the Internet. Accordingly, smaller and larger-capacity magnetic recorders, typified by HDDs, have been increasingly demanded to store such a large amount of information. A surface recording density indicative of recordable information content per unit recording area is required to be higher to realize a smaller and larger-capacity HDD.
At present, the surface recording density improves at an annual rate of about 100%. The increased surface recording density accordingly reduces an area on a record carrier corresponding to one bit as a minimum unit of magnetic record data, and a signal magnetic field from the record carrier becomes weaker as a matter of course. A small and highly sensitive read head is required to faithfully read such a weak signal magnetic field with certainty.
The current surface recording density of 20-30 Mb/in2 requires a magnetic sensor as a read head to use a spin-valve film that exhibits a Giant Magnetoresistive (referred to as “GMR” hereinafter) effect. The spin-valve film has a sandwich structure including two uncoupled ferromagnetic layers separated by a non-magnetic metal layer, wherein one of the ferromagnetic layers has a fixed direction of magnetization.
A fixed magnetization usually attaches an antiferromagnetic metal layer, typified by Fe—Mo alloy, to one of the ferromagnetic layers. According to this structure, when an external magnetic field is applied, a direction of magnetization of an unfixed ferromagnetic layer (or free layer) freely rotates and accords with a direction of the external magnetic field, generating a difference in angle relative to the direction of magnetization of the ferromagnetic layer (or pinned layer) having the fixed direction of magnetization. The spin dependent scattering of a conduction electron varies according to the difference in angle, and thus the electric resistance value changes. A state of an external magnetic field or a signal magnetic field from a magnetic record carrier is obtained by detecting a changed electric resistance value.
Currently available read heads having a spin-valve film are heads of a so-called Current in Plane (referred to as “CIP” hereinafter) configuration that applies the sense current parallel to laminated surfaces in the spin-valve film. With higher surface recording density, a read-head size should be reduced according to a reduced bit size. This is because a read head excessively large for a record bit size would simultaneously take in magnetic information from both a target record track and track(s) adjacent to the target record track, deteriorating resolution in a direction of the track's width. The surface recording density of the 100-Gb/in2 class requires head's size in a width direction to be reduced down to about 0.1 μm. A head of the CIP configuration needs a reduced device's height with a decreased device's width. This is because a weak signal magnetic field would result in such a small influential depth of the signal magnetic field in a direction of the device's height that no magnetic reversal occurs and no magnetic resistance appears at an end of the spin-valve film distant from the record carrier, and this dead section deteriorates the sensor's sensitivity.
Therefore, the read head of the CIP configuration needs to reduce the device's width as well as the device's height with higher surface recording density, and device resistance itself seldom changes with an increase of the surface recording density. However, an application of the same sense current increases the current density for the smaller device height, possibly deteriorating the device due to migration, etc. Thus, the sense current should be small with an increase of the surface recording density or with a decrease of the device's height, and an available readout output becomes small in proportion to the decreased sense current.
On the other hand, there has been proposed a read head of a configuration that applies the sense current perpendicular to laminated surfaces in a read sensor film. This is called a head of a Current Perpendicular to Plane (referred to as “CPP” hereinafter) configuration from a direction of an application of the sense current. The GMR sensor using the CPP configuration may be called a CPP-GMR.
This configuration effectively draws out a scattering effect, as one of the factors for causing a change in magnetic resistance, dependent upon a spin direction of a conduction electron at an interface between magnetic and non-magnetic laminated layers, and improves a rate of change in magnetic resistance about twice as high as the CIP configuration. Therefore, this configuration is expected to realize an extremely sensitive read head. In addition, the device sectional area (device's width×device's height) through which the sense current flows is three to five times as large as CIP 's sectional area (device's width×device's height). Moreover, the CIP configuration has different specific resistance for each metal layer in the laminated film and the current flows in such a layer as a Cu layer having small specific resistance. Therefore, if it is assumed that the maximum current density is the current density at the part where the most current flows, the film as a whole allows the sense current of only 40% of the permissible current density. Therefore, the CPP configuration may have a flow of the sense current 7-12 times as large as the CIP configuration. In addition, it is necessary to reduce the sense current value due to the reduced device sectional area with the higher surface recording density, but a readout output value becomes advantageously constant when a rate of change in resistance is constant since the device's resistance increases in inverse proportion to the device sectional area.
Hitherto, no suitable materials for a CPP-GMR sensor have been proposed which have a reduced hysteresis and sufficient control over magnetic domain as well as providing large changes in the magnetic resistance. Undesirably, the unstable reduction of hysterisis and control over the magnetic domain make unstable operations of a magnetic head to which the CPP-GMR sensor is applied, while unsatisfactory changes in magnetic resistance would lower an output from the magnetic head (or sensitivity).
Accordingly, the instant inventors have initially reviewed use of a multilayer GMR film as materials for CPP-GMR sensor, and determined that the multilayer GMR film is not so suitable because the multilayer GMR film has such a disadvantageous structure that it is difficult to reduce the hysteresis and to control the magnetic domain for stable operations of the magnetic read head, etc.
The instant inventors have then reviewed an application of a spin-valve film to the CPP structure as the CIP head. Although a usual spin-valve film advantageously facilitates control over the magnetic domain, it does not disadvantageously provide large magnetoresistive changes. Thus, the usual spin-valve film is of low practical use.